G. Dellepiane

Normal Coordinate Calculations as a Tool for Vibrational Assignments. I. Fundamental Vibrations of Simple Aliphatic Amines

Fundamental frequencies, normal coordinates, and force constants were calculated using an Overlay technique for CH3NH2, CD3NH2, CH3ND2, CD3ND2, (CH3)2NH, (CH3)2ND, (CH3)3N, and (CD3)3N. Both valence and Urey–Bradley force fields were adopted. The vibrational assignments previously reported on all these molecules have been partly confirmed and partly modified. The usefulness and limitations of normal coordinate calculations in problems of vibrational assignments is discussed.

Fermi resonances in solid N‐methyl‐acetamide

The analysis of the low temperature vibrational spectrum of N‐methyl acetamide recorded in our laboratory, has shown new features in the amide I–amide II and in the N–H stretching regions. In this paper an interpretation of the experimental data in terms of Fermi resonances is reported. For this purpose a theoretical treatment has been worked out and formulas are presented which allow the calculation of both intensities and frequencies in terms of anharmonic coupling constants. A satisfactory agreement between calculated and experimental data has been found.

A note on the correlation between infrared frequences and hydrogen-bond lengths in hydroxyl chains

Abstract An experimental relationship between the Oue5f8H stretching frequencies and the O…O distances is obtained for a selected class of chemically similar hydrogen-bonded compounds. The variation of the anharmonic frequencies with temperature in these crystals is discussed in terms of the data reported.

A study of the conformational analysis and intermolecular hydrogen-bonding in solid malonaldehyde by infrared spectra

Abstract The low-temperature infrared spectrum of solid malonaldehyde (MA) is analyzed and discussed. It is shown that the open chain s-trans conformation is stabilized by the formation of a strong intermolecular H-bond.

Normal coordinates calculation and valence force field for propyne, propargyl halides, butyne‐2, halobutyne‐2, and 1,4‐dihalobutyne‐2

Force constants and normal modes were evaluated using an overlay technique for CH3CCH, CH3CCD, CD3CCH, CD3CCD, CH2FCCH, CH2ClCCH, CH2BrCCH CH3CCCH3, CH3CCCH2Cl, CH3CCCH2Br, CH2FCCCH2F, CH2ClCCCH2Cl and CH2BrCCCH2Br. Particular attention has been given to diagonal and off‐diagonal force constants related to linear bending coordinates. The force field reported in this paper gives such a satisfactory frequency fit that it should provide a good starting point for the explanation of the peculiar band widths of 1,4‐dihalobutyne‐2.

Polarized resonant Raman scattering of cis polyacetylene

Polarized resonance Raman measurements of highly oriented films of cis polyacetylene have been carried out. The intensities of the three Raman fundamentals in the four different scattering configurations have been measured and corrected for the strong anisotropy of the optical properties. The experimental data are indicative of an almost perfect alignment of the cis polyenic chains along the stretching direction. Preliminary measurements of the depolarization ratios carried out on isotropic samples in the resonance region appear to be inconsistent with the view that the visible band is dominated by a single transition polarized along the chain axis.

Vibrational spectrum and hydrogen bonding in solid α-naphthol

Abstract The i.r. spectrum of α-naphthol has been obtained at different temperatures and at different degrees of deuteration. A method is proposed for determining the orientation of the crystallographic axis with respect to the incident beam. The absorption bands due to hydrogen bonded groups are then analysed in terms of a simple dipole-dipole model.

Depolarization ratios of the resonance Raman bands of soluble trans‐polyacetylene

Depolarization ratios for the totally symmetric Raman lines in soluble polyacetylene have been measured for various exciting wavelengths in the resonance region. The origin of the unusually high values of the depolarization ratios and their variations with the exciting wavelength are discussed in terms of two overlapping electronic transitions in the visible region.

Chemical modulation of the electronic properties of polydiacetylenes

We have investigated the effects of the chemical substitution of long alkyl chains in the carbazolyl group of a polycarbazolyldiacetylene on its electronic properties. The electronic absorption and Raman spectra of the unsubstituted unsoluble polycarbazolyldiacetylene polyDCHD and those of the alkyl-substituted soluble polymers (polyDCHD-S, polyDCHD-HS) in the blue form are very similar indicating that the backbone conjugation length is practically not affected by this substitution. Very different instead is the nature of the long-lived photoinduced excitations of these polymers. Charged carriers as well as triplet excitons are formed in the unsubstituted blue polymer while triplet excitons dominate the photoinduced spectrum when long alkyl chains are attached to the aromatic ring. Only triplet excitons are observed in the red form of the soluble polydiacetylenes. We have interpreted these findings as due to the different molecular and supramolecular structure of these polymers. The larger interchain separation achieved in the substituted polycarbazolyldiacetylenes favours the photogeneration of triplet excitons relative to charged species. This effect is even more pronounced for the red form of these polymers where the twisted conformation further decreases the interchain coupling. The femtosecond‐picosecond time evolution of the excited state dynamics of the red form of polyDCHD-S and -HS in a polyethylene matrix has also been measured by the pump-probe technique. Films of this type in which the polymer backbones of the different chains are isolated and in the twisted conformation are particularly suitable for this investigation. We believe in fact that these conditions can allow a meaningful comparison of the experimental data with the results of highly correlated quantum mechanical calculations carried out on oligomers. The transient differential transmission spectra display characteristic features depending upon the probe time delay. Photobleaching at 2.3 eV and photoinduced absorptions at 1.8 and 1.5 eV are present at short probe delay. The 1.5 eV feature, present also at large probe delays, is assigned to triplets which are formed by singlet fission as a pair with an overall zero spin. These pairs, when survived to their recombination and trapped in defect sites, can live up to milliseconds. On the basis of the quantum mechanical calculations on oligomers, we believe that the state responsible for the fission is the optically forbidden 2 1 Ag level below the first allowed 1 1 Bu state. q 2000 Elsevier Science B.V. All rights reserved.

Semiempirical potentials and intramolecular forces from torsional data: Part I. Threefold symmetric Potentials☆

Abstract Torsional energy levels experimentally determined from microwave and infrared spectra are compared with those obtained from torsional potential functions derived from semiempirical nonbonded interaction potentials. A method is presented for the calculation of torsional energy levels from semiempirical potentials. A new conformational potential energy function which reproduces both torsional frequencies and experimental barrier height is proposed and discussed for molecules which contain Hue5f8H and Hue5f8F nonbonded interactions. It is shown that forces and force constants between nonbonded atoms can be calculated from the spectroscopic potential function just determined. The obtained results are more meaningful than those obtained for the same nonbonded interactions from the Urey-Bradley force field.